1. Field of the Invention
The present invention relates to a process to transmit data between a read/write device and a transponder. Systems consisting of read/write devices and transponders are used for the contactless identification of objects, persons and animals. The transponder is a data memory whose information can be read out and changed remotely with the read/write device. The information can be transmitted magnetically or electromagnetically. The data is transmitted from the read/write device to the transponder by changing the field strength of a high-frequency carrier and evaluating the time between sequential field strength changes.
2. The Prior Art
In the development of transponder systems, great value is placed on using as little energy as possible and integrating as many components as possible in a single chip. The first requirement for active transponder systems, having their own energy supply is to achieve the longest possible battery life. Passive transponder systems receive their energy from the magnetic or electromagnetic field of the read/write device. In these systems, the goal is to attain the longest possible range. This second requirement is also important due to the size of the transponder and the manufacturing costs.
According to the state of the art, a RC oscillator can be integrated on a single silicon chip with all of its frequency-determining components. Due to the limitations of the manufacturing process and the typical operating temperature and voltage range, this RC oscillator has a frequency tolerance of up to +/-30% in reference to its nominal value dimensioned in the chip design.
Unfortunately, when the time base for evaluating the coded-information is based on the frequency of such an oscillator, the possible data transmission speed is limited. To clearly differentiate between the different times between the field strength changes of the carrier, the time differences must be at least bigger than the possible tolerance of the oscillator. This requirement correspondingly means that there are slow data rates to prevent the tolerances of the oscillator from distorting the decoding.
To calibrate the oscillator, the oscillator can be trimmed with a laser beam after chip production or adjusted with memory cells. This process is involved and also has the disadvantage that frequency tolerances cannot be compensated by temperature and voltage changes.